Semiconductor-producing apparatuses are used to produce semiconductor devices. For example, a reduction projection type sequential exposure apparatus (hereinafter appropriately referred to as “stepper”) is exemplified as a representative semiconductor-producing apparatus. The stepper is equipped with a highly accurate projection lens. In order to guarantee the accuracy of the highly accurate projection lens, it is necessary to measure the transmission wavefront aberration or the reflection wavefront aberration for the entire projection lens and each of optical elements for constructing the projection lens in the actual exposure wavelength area. For this purpose, a variety of interferometers have been suggested, each of which uses a light source having high coherence for emitting a light beam having a wavelength which is the same as that included in the exposure wavelength area or which is substantially equal to that included in the exposure wavelength area.
The high degree of integration of the semiconductor device is advanced in recent years. In order to respond to the advance of the high degree of integration, the exposure wavelength of the stepper is shortened. For example, the wavelength is shortened to the i-ray (λ=365 nm) from the g-ray (λ=436 nm) which is based on the use of the high pressure mercury lamp as a light source. Further, the wavelength is shortened to the ArF excimer laser (λ=193 nm) from the KrF excimer laser (λ=248 nm). As a result, it is extremely difficult to obtain an available light source which has high coherence and which has an oscillation wavelength in the vicinity of the exposure wavelength. Therefore, a point diffraction interference measuring apparatus has been suggested, which is based on the use of the point diffraction interferometry capable of performing highly accurate interferometry or interference measurement even by using a light source having relatively low coherence.
Such a conventional point diffraction interference measuring apparatus will be explained below with reference to an accompanying drawing. FIG. 8 shows a schematic arrangement of the conventional point diffraction interference measuring apparatus. In FIG. 8, a light flux, which is emitted from a light source 1, illuminates a pinhole 2. The light beam, which has outgone from the pinhole 2, can be regarded as an approximately ideal spherical wave. The spherical wave, which has outgone from the pinhole 2, passes across a collimator lens 3, a bending mirror 4, a beam splitter 5, a bending mirror 6, a bending mirror 7, and a light-collecting lens 8 successively in this order from the side of the light source 1. The light beam, which has outgone from the light-collecting lens 8, is transmitted through a test sample 9, and the light beam is reflected by a bending mirror 10.
The reflected light beam again passes across the test sample 9, the light-collecting lens 8, the bending mirror 7, and the bending mirror 6 successively in this order from the side of the bending mirror 10. The light beam is reflected by the beam splitter 5 toward a light-collecting lens 11. The light beam, which has been reflected by the beam splitter 5, comes into a diffraction grating 12 after passing through the light-collecting lens 11.
The light beam, which has come into the diffraction grating 12, is diffracted by the diffraction grating 12 into a 0-order light beam, a +1-order light beam, and diffracted light beams having other orders. The 0-order light beam comes into a pinhole 13a which is provided in order to generate a reference light beam. The light beam, which has outgone from the pinhole 13a, can be regarded as an approximately ideal spherical wave. Therefore, this light beam serves as the reference light beam. The +1-order light beam comes into a window 13b which is provided in order to allow a measuring light beam to pass therethrough. The reference light beam which has outgone from the pinhole 13a and the measuring light beam which has outgone from the window 13b pass through a collimator lens 14, and they come into an interference fringe-detecting section 15. Interference fringes, which are formed by the interference caused between the reference light beam and the measuring light beam, can be observed with the interference fringe-detecting section 15.
The conventional point diffraction interference measuring apparatus described above uses the diffraction grating in order to separate the light flux. Therefore, a problem arises such that the diffracted light beams having the orders other than those of the 0-order and +1-order light beams come into the pinhole 13a and the window 13b to generate any noise light beam.
Further, the 0-order spot light beam coming into the pinhole 13a and the +1-order spot light beam coming into the window 13b are widened or spread due to the aberration respectively. Therefore, a problem arises such that the spot light beams are hardly separated from each other, and they mutually behave as noise light beams. The presence of the noise light beams as described above has lowered S/N or the dynamic range during the detection of the interference fringes.
The light flux, which comes into the pinhole as described above, is converted into the approximately ideal spherical wave by passing through the pinhole. It is noted that the light amount of the generated spherical wave differs depending on the aberration of the light flux coming into the pinhole. The light amount of the light flux coming into the pinhole is determined by the design of the diffraction grating. When the aberration of the light flux coming into the pinhole is small, and the light-collecting performance is satisfactory, then the light amount of the light beam passing through the pinhole is large, and hence the light amount of the reference light beam (generated spherical wave) is increased. On the other hand, when the aberration of the light flux coming into the pinhole is large, and the light-collecting performance is unsatisfactory, then the light amount of the light beam passing through the pinhole is small, and hence the light amount of the reference light beam (generated spherical wave) is decreased. Therefore, a problem arises such that any difference appears between the light amount of the reference light beam and the light amount of the measuring light beam, and the contrast of the interference fringes is consequently lowered.
Further, every time when the test sample is replaced with another test sample to perform the measurement, it is necessary that the test sample is subjected to the alignment (hereinafter appropriately referred to as “alignment for the test sample”) with respect to the incoming or incident light beam coming into the test sample in order to allow the light beam having passed through the test sample to return to the pinhole, for the following reason. That is, the test sample represents a variety of optical members which involve different amounts of generated aberration respectively. Therefore, the light flux, which has passed through the test sample as described above, does not necessarily come into the pinhole, because the optical path, which is directed toward the pinhole, is changed due to the aberration and the refraction. For this reason, it is necessary that the test sample is subjected to the alignment three-dimensionally with respect to the incoming light beam so that the light beam, which has passed through the test sample, is accurately introduced into the minute pinhole. However, a problem arises such that it is extremely difficult to perform the three-dimensional alignment accurately in a short period of time. In this specification, the phrase “alignment for the test sample” is in a concept which includes not only the adjustment of the orientation or the position of the test sample with respect to the measuring light beam but also the adjustment of the angle or the position of the reflecting mirror which is provided in order to allow the light beam having passed through the test sample to return to the test sample.